1. Field of the Invention
This invention relates to a magneto-optical recording medium suitable as an information recording medium exclusively for reproduction, a method of manufacturing the same and a method of reproducing information from the magneto-optical recording medium.
2. Related Background Art
In recent years, attention has been paid to a magneto-optical disc as a high-density recording medium capable of rewriting. Demand for further enhancing the recording density of the magneto-optical disc to thereby provide a recording medium of large capacity has been increasing.
A linear recording density of an optical disc depends greatly on the laser wavelength .lambda. of a reproducing optical system and the numerical aperture NA of an objective lens, and as regards the spatial frequency during signal reproduction, the order of NA/.lambda. is the detectable limit. Accordingly, to realize high density in the conventional optical disc, it is necessary to shorten the laser wavelength .lambda. of the reproducing optical system and to increase the numerical aperture NA of the objective lens. However, improvement in the laser wavelength and the numerical aperture of the objective lens is limited. Therefore, several techniques of contriving the construction of a recording medium and a reading method and improving the recording density have been proposed.
For example, the inventors have proposed, in Japanese Laid-Open Patent Application No. 6-290496, a magneto-optical recording medium capable of reproducing signals of periods below the diffraction limit of light at a high speed without reducing the amplitude of a reproduction signal, and a method of and apparatus for reproducing the same. That is, according to the above-mentioned Japanese Laid-Open Patent Application No. 6-290496, the magneto-optical recording medium is a magneto-optical recording medium comprising at least first, second and third magnetic layers laminated in succession, and more particularly is a perpendicular magnetic recording medium characterized in that the first magnetic layer is a perpendicular magnetization film (displacement layer) which is relatively small in magnetic wall coercivity and great in the degree of displacement of the magnetic wall in the vicinity of the ambient temperature as compared with the third magnetic layer, the second magnetic layer comprises a magnetic layer (switching layer) lower in Curie temperature than the first magnetic layer and the third magnetic layer, and the third magnetic layer is an ordinary perpendicular magnetization film (memory layer) excellent in preserving the stability of a magnetic domain. The method of reproducing this magneto-optical recording medium is characterized by applying a light beam from the first magnetic layer side while moving it relative to said medium, forming on said medium a temperature distribution having a gradient relative to the movement direction of the spot of said light beam, making a part of said temperature distribution into a temperature area higher than the Curie temperature of the second magnetic layer to thereby displace the magnetic wall formed in the first magnetic layer, detecting a change in the plane of polarization of the reflected light of the light beam and reproducing recorded information.
However, according to said magneto-optical recording medium and the reproducing method therefor (Japanese Laid-Open Patent Application No. 6-290406), it has become possible to reproduce signals of periods below the diffraction limit of light at a high speed without reducing the amplitude of a reproduction signal, but the range to which this technique is applicable is limited to a magneto-optical recording medium capable of effecting both recording and reproduction. That is, it cannot yet be applied to a magneto-optical recording medium exclusively for reproduction.
On the other hand, the manufacture of an optical type information recording medium exclusively for reproduction is usually done by a method of using an original disc on which information is recorded by forming projections and recesses, and transferring the projections and recesses onto a plastic substrate. This method is very well suited for mass production. This magneto-optical recording medium exclusively for reproduction is widely used as a medium for distributing music information, movie information, software, etc., and the larger capacity thereof is desired.